The easiest way is perhaps to use the DIV=<value> option for the document class, see table 2.1 in section 2.6 Options and Macros to Influence the Page Layout in the (English version of) the KOMA-script manual (dated 2013-12-19). You can choose integers from 6 to 16, where larger numbers give a larger type area, i.e. smaller margins.

For example

\documentclass[fontsize=13pt,DIV=14]{scrreprt}

Browsing the manual, I also see that you can set the type area with

\areaset[BCOR]{Width}{Height}

where BCOR is binding correction. Read chapter 2 Construction of the page layout with typearea of KOMA script's manual for more information.